Devices for transmitting communication signals, such as radio frequency (RF) communication signals, must regulate the power at which the signals are transmitted. For example, a conventional cellular telephone, such as a class III portable as defined under the IS-19 standard published by the Electronics Industry Association/Telecommunications Industry Association (EIA/TIA) located at 2001 Pennsylvania Ave., N.W. Washington, D.C. 20006, is adapted to transmit RF signals at one of six predetermined power levels varying at 4 dB steps. The more recent IS-136 standard, also published by the EIA/TIA, calls for a class IV portable which is required to have three additional power steps below the step 7 level of class III portables. Each of these new levels is also a 4 dB step, requiring a power control dynamic range increase from 20 to 32 dB.
Conventional radio designs use a coupler with a peak detector on the coupled port to detect radio power output for power setting purposes. The peak detector operates with a minimal bias voltage. At higher power out levels, the detector output voltage linearly tracks the power into the peak detector. As power falls, the detect output voltage approaches its bias value asymptotically.
As a result, the power/detector output voltage curve no longer follows the linear slope. Also, larger changes in power are required to produce each dB of change in detector output voltage, resulting in a loss of resolution as the detector output voltage approaches its bias value.
Conventional radio designs also typically set the coupling ratio to provide the minimum necessary power into the detector at the lowest power step in order to keep the detector insertion loss to a minimum. Applying this technique to a class IV portable would require a 12 dB increase in coupling ratio to maintain equal performance. With existing couplers operating in the 16 to 20 dB range for coupling, such an approach would require a detector with a 4 to 8dB coupling ratio. Such a coupler would have an unacceptably high insertion loss.
Accordingly, there is a need for a method and apparatus for detecting communication signals over a wider dynamic range.